Displacement response to axial cycling of piles driven in sand

A review of the load applied to multi-pile offshore wind turbine foundations is presented, from which the need to consider the response to axial cyclic loading is emphasised. The paucity of available data on field tests on driven piles in sand is noted. A comprehensive data set of multiple axial cyclic and static tests conducted on seven industrial-scale steel pipe-piles at a marine sand site in Dunkerque, France, is re-examined in this paper. The effects of cycling on axial capacity are interpreted by reference to stable, metastable or unstable zones defined in a normalised cyclic stability interaction diagram. A detailed analysis is made of the load–displacement and stiffness response associated with each mode of cycling. It is shown that in all cases the piles' cyclic stiffnesses show only minor changes until cyclic failure is approached. The patterns of permanent cyclic strain accumulation are sensitive to the applied mean and cyclic loading levels. Whereas displacements accumulate rapidly over just a few cycles in the unstable zone, extended cycling in the stable zone leads to minimal accumulated displacements and constant transient cyclic displacements. </jats:p

Investigating soil-water retention characteristics at high suctions using Relative Humidity control

A technique for controlling relative humidity (RH) is presented, which involves supplying a sealed chamber with a continuous flow of air at a computer-regulated RH. The desired value of RH is achieved by mixing dry and wet air at appropriate volumes and is measured for servo-control at three locations in the chamber with capacitive RH sensors and checked with a sensitive VAISALA sensor. The setup is capable of controlling RH steadily and continuously with a deviation of less than 0.2% RH. The technique was adopted to determine wetting soil-water retention curves (SWRC) of statically compacted London Clay, under both free-swelling and constant volume conditions. The RH within the chamber was increased in a step-wise fashion, with each step maintained until vapour equilibrium between the chamber atmosphere and the soil samples was established. Independent filter paper measurements further validate the method, while the obtained retention curves complement those available in the literature for lower ranges of suction

The use of kinematic hardening models for predicting tunnelling-induced ground movements in London Clay

The use of a kinematic hardening soil model for predicting short- and long-term ground movements due to tunnelling in London Clay is investigated. The model is calibrated against oedometer and triaxial tests on intact samples from different units of the London Clay. The calibrated model is then used in finite-element analysis to simulate the field response at St James's Park during excavation of the Jubilee Line Extension tunnels. The finite-element predictions compare well with the available field monitoring data. The importance of using consistent initial conditions for this complex boundary value problem in conjunction with the model parameters selected is highlighted. The stiffness response of different regions of the finite-element mesh indicates that the rate at which the stiffness degrades and the stiffness response further away from the tunnel boundary affect the short-term predictions significantly. The long-term predictions confirm that the compression characteristics of the soil control the magnitude of the consolidation settlements and its permeability the shape of the long-term settlement profiles

Investigating the effect of tunnelling on existing tunnels

A major research project investigating the effect of tunnelling on existing tunnels has been completed at Imperial College London. This subject is always of great concern during the planning and execution of underground tunnelling works in the urban environment. Many cities already have extensive existing tunnel networks and so it is necessary to construct new tunnels at a level beneath them. The associated deformations that take place during tunnelling have to be carefully assessed and their impact on the existing tunnels estimated. Of particular concern is the serviceability of tunnels used for underground trains where the kinematic envelope must not be impinged upon. The new Crossrail transport line under construction in London passes beneath numerous tunnels including a number of those forming part of the London Underground networ

Numerical investigation of the effects of tunnelling on existing tunnels

Construction of the Crossrail tunnels just beneath the existing Central line tunnels at the northern side of Hyde Park provided the impetus for this paper. A basic three-dimensional (3D) finite-element (FE) model was developed to study a general case of a new tunnel (NT) crossing perpendicularly below an existing tunnel (ET). A series of 3D FE analyses was carried out and the results presented in this paper reveal some of the interaction effects. Changes in hoop forces, bending moments and lining deformations of the ET due to excavation of the NT are discussed. Conclusions are drawn about how the relative position of the excavation face of the NT in relation to the ET's axis affects the latter's behaviour. Cross-sectional and longitudinal deformations of the ET are discussed, leading to recommendations for field monitoring of similar interaction cases. Two parametric studies were also carried out to quantify the effects of the magnitude of the earth pressure balance machine face pressure and the longitudinal stiffness of the ET on the predicted behaviour of the ET due to construction of the NT

Swelling behaviour of an expansive clay at high suction

Deep geological disposal designs for nuclear waste often include an engineered barrier to protect the waste canisters and prevent leakage . The long - term safety of the re pository relies on studies of the buffer material . Oedometer tests provide values of de sign parameters for numerical simulations. A newly - developed oedometer with automated suction control is presented to assist in the investigation of the coupled hydro - me- chanical - volumetric behaviour of an expansive clay, namely a natural sodium bentonite. The displacement - controlled device was developed to apply suction over a range of 1 0 MPa to 300 MPa using a divided - flow humidity - generator. The device allows the application of combined stress and suction states, and continuous stress paths of constant vo lume, stress or suction. The development of the new oedometer is described. Results obtained during the preliminary tests are evaluated through comparison with experimental data from similar tests found in the literature. The current method benefits from c ontinuous control of suction with servo - control of relative humidity using calibrated capacitance hygrometers. The system self - compensates for minor temper- ature changes and therefore the requirement for thermal insulation is not as crucial as in vapour equ ilibrium methods

Predicting tunnelling-induced ground movements and interpreting field measurements using numerical analysis: Crossrail case study at Hyde Park

Ground response to the construction of the Crossrail tunnels in London Clay beneath Hyde Park has been modelled numerically using advanced finite-element analyses. The soil model used for modelling the London Clay was a kinematic hardening soil model (named M2-SKH). This model, when used for the St James's Park greenfield site, provided excellent predictions of tunnelling-induced ground movements. Comparison of the results from the analysis of the Hyde Park greenfield site with associated field monitoring data also suggests excellent predictions, even though in this case the tunnels were: of larger diameter; deeper in the London Clay; and constructed with earth-pressure-balance machines. The influence of lining permeability was found to influence significantly short- and longer-term predictions. Interpretation of the predicted surface and subsurface vertical and horizontal displacements due to the construction of the Crossrail tunnels exemplifies how numerical analysis can assist in explaining and identifying potential ambiguities in field measurements

Apparatus for centrifuge modelling of twin-tunnel construction

In urban areas it is common for pairs of tunnels to be used as a method for building rapid transit systems. Driven by an increasing population and demand for services, tunnels are more widespread in their use than at any previous time. Construction of any form of tunnel causes ground movements which have the potential to damage existing surface and sub-surface structures. Modern tunnelling practice aims to reduce these movements to a minimum but there is still a requirement for accurate assessments of possible damage to structures resulting from settlements. For tunnels driven in clay, superposition of settlement predictions made by considering a single tunnel is an accepted method used to estimate movements around pairs of tunnels. Previous research, particularly numerical studies, has indicated that this may not necessarily be sufficient. In this paper a series of centrifuge model tests designed to investigate settlements related to twin-tunnel construction are described. The development of the experimental apparatus for sequential twin-tunnel construction with variable centre-to-centre spacing and volume loss is described in detail

Estimating column diamters in jet-grouting processes

Jet grouting is widely used in geotechnical engineering for a variety of applications and is a well-proven technique. As with many techniques developed from a practical perspective, there is still scope for improvements both in construction practice and design. This paper focuses on one of the most crucial elements of quality control required with jet grouting operations, the diameter of the constructed column. First, the jet grouting method is explained and the main issues of the concept highlighted. A description follows of the techniques available for estimating column diameter, discussing their application and evaluation on site. There is a particular emphasis on two methods: inclined core drilling and a newly-developed thermic approach (Meinhard et al., 2010) both of which were implemented on two construction projects. Data from these case studies are reported and analysed extensively, in conjunction with influencing factors such as the ground conditions, to assess their effects on the achieved diameter. An empirical approach for evaluating the diameter of jet-grouted columns is developed based on various factors influencing their size such as monitor lifting speed and soil strength

Older employees' declining attitudes over 20 years and across classes

British employers, under increasing competitive pressures, and applying new technology and work organization, have sought to reduce labour costs, resulting in work intensification and precarity. Older employees as a result are exposed to work demands that conflict with expectations of favourable treatment in late career. National survey data for Britain in the years 1992, 2001, 2006 and 2012 demonstrate a decline in overall job attitude among older employees following the changed conditions of the 1990s and across the major recession that began in 2008. To assess whether this decline is unequally distributed, decomposition by socio-economic class is carried out. This shows that older employees in the ‘service class’ of managerial and professional employees are affected at least as much as older employees in intermediate and less-skilled classes, thus underlining the age effect and showing that ‘service-class’ employees are not invulnerable to a changing economic environment